harisimrat

I have heard of Saas Giraas simran. What is the historical evidence or origins of this? Some people use "Saas Giraas simran" and "Swaas Swaas simran" interchangeably. Commonly people say "Saas Giraas", but "Saas" = breath, "Giraas" = morsel of food... i.e. to remember God with every breath...